(1) Field of the Invention
This invention relates to a process for producing a stable aqueous polymer emulsion from a radically polymerizable monomer or monomers having an unsaturated bond. More particularly, this invention relates to a process for producing a stable aqueous polymer emulsion wherein a radically polymerizable monomer or monomers are polymerized in an aqueous medium in the presence of a water-soluble copolymer which is derived from a combination of (a) an ethylenically unsaturated nitrogen-containing compound with (b-1) an unsaturated carboxylic acid or its salt and/or (b-2) an ethylenically unsaturated sulfonic acid or its salt.
(2) Description of the Prior Art
Emulsion polymerization techniques have been popularly employed for the production of various industrially important polymers such as styrene-butadiene copolymer rubber, acrylonitrile-butadiene copolymer rubber, polystyrene, polychloroprene and styrene-acrylonitrile copolymer resins, and also for the production of various polymer emulsions used for paints and adhesives, such as acrylic polymer emulsions, vinyl acetate polymer emulsions and styrene-acrylic polymer emulsions. In conventional emulsion polymerization techniques, a monomer or monomers are dissolved or dispersed in water with one or more anionic, cationic or nonionic surface active agents, and polymerized by using a water-soluble or oil-soluble initiator.
The conventional emulsion polymerization techniques have some problems. For example, when a polymer is recovered from the reaction mixture, the emulsifier used remains partially in the polymer as an impurity and has a harmful influence on the polymer. For the production of polymer emulsions used for paints and adhesives, an emulsion stabilizer must be used in order to prevent or minimize the formation of coagulated products during the emulsion polymerization and to insure that the emulsion polymerization is stably carried out. It is desired that an emulsifier be used in a minimum amount so that the remaining emulsifier does not influence to a significant extent the performance of the polymer articles, such as films. Namely, when the polymer emulsion is used as a paint vehicle, bubbles are formed in the step of paint preparation and in the step of painting, which creates a problem in that it is very difficult to work with such a paint. If an antifoaming agent is incorporated in the polymer emulsion, which is a common procedure, the resulting polymer film becomes poor in water resistance and tends to blister. In an extreme case, when a salient amount of the emulsifier remains therein in the polymer emulsion, the coating or adhesive is re-emulsified upon contact with water.
In recent years, a polymer emulsion has been used as reinforcing materials for concrete. However, when the polymer emulsion is incorporated with a cement or mortar, bubbles tend to be formed due to the emulsifier. Furthermore, the polymer emulsion reacts with multivalent metal ions, such as Ca.sup.++ and Al.sup.+++, released from the cement or mortar and, consequently, the emulsion particles are coagulated, leading to the reduction in the strength of the resulting reinforced concrete structures.
In order to overcome the above-mentioned defects, some attempts have been proposed. For example, Japanese Patent Publication No. 6,392/1963 teaches that, when emulsion polymerization of acrylonitrile is effected in the presence of a water-soluble copolymer comprised of sodium vinyl benzene sulfonate and an unsaturated vinyl compound, instead of the conventional emulsifier, the rate of polymerization is enhanced and the slurry of the polymer becomes stable, because such a water-soluble copolymer exhibits a prominent emulsifying function. However, in this proposed technique, the allowable polymerization conditions are restricted, it is difficult to suppress the formation of coagulated products in the polymer emulsion and it is also difficult to obtain a polymer emulsion having a high concentration.
Japanese Patent Publication No. 606/1972 teaches the use of a styrene/methacrylic acid copolymer, a styrene/acrylonitrile/acrylic acid copolymer or a methyl acrylate/acrylic acid/itaconic acid copolymer instead of the conventional emulsifier in emulsion polymerization. However, it is still difficult to obtain a polymer emulsion having a high concentration as well as to suppress the formation of coagulated products in the polymer emulsion to the desired extent.
Furthermore, Japanese Patent Publication No. 31,280/1974 teaches the use of a sodium styrenesulfonate/methacrylic acid copolymer instead of the conventional emulsifier in emulsion polymerization. It is stated in this reference that the polymer particles in the resulting polymer emulsion are very fine. However, the polymer emulsions are liable to be easily destroyed by multivalent metal ions, particularly the aluminum ion. Also, the pH of the polymerization mixture is restricted to below 4.